The invention relates to a multy-party telephone system for connecting, by a small number of wires, telephone sets included in the system to at least one external line and to each other respectively, all with signalling between the telephone sets and an exchange unit being carried out by digital signal words.
In the market there is a great demand for telephone systems in which telephone sets are included for connection to external lines and with internal connection possibilities to each other. These systems can either be connected as sub systems to a local exchange or be executed as separate plants the output lines of which then are connected to a local exchange. Characterizing for these systems are that they as a rule are traffic intensitive and have a relatively large number of coupling services which can easily and quickly be connected with for example buttons on the telephone set. Furthermore, it is important in these systems to have an effective carry and attendance service function.
In earlier known solutions a great number of wire pairs are usually required in the internal network of the system. Certain tests have been made to eliminate the number of wire pairs for example as shown in Swedish Patent application No. 7503102-1. However, in the arrangement according to that application there is an inconvenience in that separate signal generators have to be used for internal signalling between connected telephone sets and when signalling towards external lines.
In order to obtain safe trunk discrimination control when utilizing a separate generator for the external selection signalling this signal must be repeated in the central unit of the system. Furthermore, on internal call no call secrecy is obtained when all the calls are exchanged through a common line.
The invention which solves said problem is characterized in the appended claims.